The northbound bridge over De Neveu Creek near Fond du Lac, Wisconsin, looks just like the two-lane southbound crossing, which is made conventionally from concrete poured over steel rebar. But in 75 years the difference will be clear. Inside the northbound bridge’s concrete deck, builders used a prefabricated plastic grid instead of steel. Engineers at the University of Wisconsin†Madison who tested the fiber-reinforced polymer bridge expect it to last at least twice as long as its steel counterpart because it resists corrosion from the salt and water that seeps through surface cracks in the concrete. In icy states, the durability is worth the 35 percent higher cost.